The retrovirus-like transposon gypsy has been shown to account for high rates of mutation in some strains of Drosophila melanogaster. This transposon is usually found in low copy number (3-4) per genome and is extremely stable in most strains. The Uc strain shows a wide range of copy number in sublines (3-60+) and when this strain is outcrossed to stable strains, some matings result in high rates of mutation associated with mobilization and insertion of gypsy into chromosomal sites previously free of gypsy. From such dysgenic crosses only the F1 female and her offspring, both male and female, show the high mutation rates. F1 males are mutationally stable and gypsy is not mobilized. Reciprocal crosses give similar results, suggesting that cytoplasmic factors are not involved in the mobilization process. Examination of polytene chromosomes from F2 and F3 offspring of dysgenic crosses show that gypsy is amplified in copy number and mobilized to insert at new chromosomal locations in somatic cells. This is consistent with the observation that most mutations transmitted through the germ cells appear in clusters. We are studying the molecular structure of gypsy to determine whether incomplete or defective copies can be mobilized when a complete copy is also present in the cell. We also are making crosses of various types to determine the genetic factors that are important in mobilization of the transposon.